UV light is used for many different purposes including, for example, the use of UVC irradiation for the purification or other treatment of fluids such as air or water. U.S. Pat. No. 6,693,382 entitled “Control system for microwave powered light sources” discloses that there is a maximum desirable power density for UVC emitting electrodeless light sources. There is also a maximum desirable bulb diameter to prevent reabsorbtion of UVC generated by a plasma core which will make the system inefficient at outputting UVC light. It is, therefore, often advantageous to maximise the amount of energisable plasma per unit of irradiator length by the use a plurality of UV bulbs in parallel.
In existing systems, such as that described in GB 2 399 216, when a plurality of bulbs are used in a single irradiator light source, light from each bulb is emitted evenly over it's circumference and, thus, part of each bulb's emission will be transmitted onto, and absorbed by, other adjacent bulbs and, thus, not all the light outputted from the bulb can be used to irradiate the fluid. Additionally, the power density of the neighbouring bulbs is also disturbed. Hence, the power per bulb length output is limited as some of the power is reabsorbed by neighbouring bulbs.
GB 2 413 005 describes an improved radiator where the centre conductor may be reflective and therefore may redirect some of the light that would otherwise be absorbed by adjacent bulbs out of the irradiator. However, the necessary shape of the centre conductor limits its ability to reflect all the light incident upon it. Additionally, if the structure is powered by microwaves from one end only it is difficult to evenly energise the bulbs within the irradiator along their entire length.
The present invention provides apparatus including an inner conductive element, a structural element, an outer conductive element and at least one bulb configured to emit light in response to microwaves. The outer conductive element is substantially transmissive to light. The structural element forms conductive cavities that are preferably longitudinal. The bulb is positioned within one of the cavities within the outer conductive element. The inner and outer conductive elements are coupled to a microwave source in such a way that the inner conductive element acts as the centre conductor of a coaxial transmission system and the outer conductive element acts as the outer conductor of the coaxial transmission system.
The structural element is preferably also conductive; thereby acting in conjunction with the outer conductive element to form an outer conductor of subsidiary coaxial systems that are formed within the cavities of the irradiator.
It is preferable that the structural element includes a bore that extends through it and through which the inner conductive element can pass from one end of the irradiator to the other.
The apparatus may be further provided with at least one chamber, but preferably two or more. The one or more chambers may be defined either by an end plate of the irradiator and the end of a section of the structural element. Alternatively, the one or more chambers may be defined by a space between two sections of inner structural element. The inner conductor extends through the chambers, and hence, a microwave cavity is created in each cavity between the outer conductive element which extends over the outside of the chamber and the inner conductive element. The elements being connected to and energised by a suitable microwave source.
Preferably, the apparatus is provided with a plurality of bulbs and the structural element is configured to provide an equal number of cavities to the number of bulbs. The bulbs extend through the cavities and at least partially into the chamber or chambers present in the apparatus.
Typically, the UV bulbs are electrodeless. Preferably, the UV bulbs have a diameter of less than 22 mm and are constructed of UVC transmissive quartz. Preferably, they contain a mixture of Argon and Mercury such that, when exposed to microwave radiation at approximately 2.45 GHz they illuminate and irradiate at the Mercury based UV spectra.
As described above, the chambers will energise the plasma in the parts of the bulb that extend into the chambers. As microwave induced plasmas such as those described above act as lossy conductors, the energisation of the plasma will extend along the longitudinal cavity with the UV bulb acting as the central conductor within the cavity. Thus, the cavities encompassed as they are by the outer conductive element and structural element form subsidiary coaxial transmission systems.
Hence, this structure enables a plurality of bulbs in cavities around a structural element can be more evenly energised by microwave energy present in the chambers.
Preferably, the surface of the cavities is reflective and optimised the maximise the UV emissions from the bulbs through the outer conductive element.
The microwave source may be introduced directly to a chamber at one end of the irradiator. Alternatively, it may be connected via a waveguide or co-axial line where the central conductor of the coaxial line is connected to the inner conductive element of the irradiator and the outer conductor of the coaxial line is connected to the outer conductive element.
Preferably, the structural is a solid electrical conductor. Preferably, the structural element is constructed from aluminium extrusion with a reflective polished outer surface extending up to the junction between an edge of the structural element and outer conductive element. This reduces the amount of light emitted by a bulb being intercepted by any other bulbs in the system.
The inner conductive element is preferably a metallic rod. The inner conductive element preferably passes through the bore of the structural element and, thus, this acts as a coaxial transmission line transmitting microwave energy from the source input to the chambers in the irradiator. This allows the bulbs within the irradiator to be energised at multiple points along their length.
Preferably, pressurised air can be passed through the space between the inner conductive element and the bore of the structural element. The air can return through the longitudinal cavities thereby cooling the bulbs or allowing the creation of ozone.
The apparatus may include a UV transmissive fluid-tight envelope arranged around the outer conductor which allows the generator to operate whilst immersed in a fluid such as water. Preferably, the envelope may be formed from quartz.
Preferably, the apparatus includes a spark generator arranged to generate a spark through or adjacent the bulb or bulbs in order to encourage ignition of the bulb or bulbs.